The site also formerly ran a solar-observing array of thirty-two 10 ft (3 m) dishes, and a single 1.8 m solar flux monitor observing at 10.7 cm wavelength. The site is currently used to provide high accuracy geodetic location information to the present day for applications such as real time GPS signal correction. The site has its own atomic clock, a standard feature for radio telescopes that can also serve to receive telemetry from Deep Space missions.

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Algonquin Radio Observatory was inaugurated in 1959 and became Canada's national radio observatory in 1962.[1] The observatory house complex, radiometer building, utility buildings, University of Toronto Laboratory, 10 m (33 ft) dish and parabolic microwave feed horn instruments were designed in 1959 and construction was completed in phases over the next several years. In 1961, the site was selected by the National Research Council of Canada as suitable for the construction of a 120 ft (37 m) fully steerable antenna.[2] By 1962, plans showed that the main instrument had grown to a 150 ft (46 m) antenna; construction of this commenced in 1964.

Prior to the construction of the ARO, Arthur Covington had been running a solar observation program at the National Research Council of Canada (NRC) Ottawa Radio Field Station.[3] The station was primarily a radar research site, and ongoing radar work interfered with the solar instrument Covington had built as a personal project. As the value of the observations became evident, the instrument was moved about five miles (8 km) away to Goth Hill, a more radio-quiet location. But as Ottawa grew this site soon started becoming radio-noisy as well, due mostly to increasing air traffic at a nearby airport. Looking to improve the quality of their measurements, they proposed building a new solar telescope located far away from built up areas. Easy access from Ottawa made Algonquin a fairly obvious choice, although it was about 200 km away the roads were good quality and easy to travel.

Construction of the solar observation site started in 1959. The first instrument on the site was a new solar telescope, similar to Covington's original 4 ft (1.2 m) instrument, but slightly enlarged to 6 ft (1.8 m) which allowed it to better observe the entire solar disk. This instrument operated in parallel to the original at Goth Hill until 1962, when it took over these duties completely.[4] A second 6 ft (1.8 m) telescope, identical to the one at ARO, was later installed at the Dominion Radio Astrophysical Observatory (DRAO) in Penticton, British Columbia as a backup.

Another solar instrument pattered on a different Goth Hill device followed, this one consisting of a series of thirty-two 10 ft (3 m) parabolic collectors connected to a common 700 ft (215 m) long waveguide. Using phased array techniques this instrument could image portions of the Sun's disk, compared to the single-dish instrument which saw the sun as a single unresolved "dot". The new instrument was up and running in 1966, adding to Covington's study of the sun by directly imaging the radio signal from sunspots and filaments.

Construction on the 150 ft (46 m) telescope started in the spring of 1964. The concrete base weighed 300 tons, the steel dish and the its rotating mount another 900 tons. An equatorial mount in the base, only five feet high, positioned the instrument. The telescope was designed to operate at higher frequencies than existing instruments, requiring much of it to be constructed of flat plates instead of an open mesh in order to accurately focus these signals. The surface was built to be accurate to 1/5 of a centimeter, allowing it to accurately focus wavelengths to around 1.5 cm. Construction was completed in early 1966, and the telescope started operations in May 1966. Work was also completed a polar mounted paraboloid microwave horn and an 11 m equatorial mount dish north of the main antenna complex.

One of the earliest extended projects carried out on the instrument was the first successful very long baseline interferometry (VLBI) experiment. Long Baseline Interferometry compares the signals from two or more telescopes, using the differences in phase between the signals to resolve the objects. Earlier experiments had used direct electrical links or microwave relays to extend the distance between the two telescopes, while still allowing real-time comparison of the phase of the two signals in a common instrument. However this limited the distance between the two instruments, to the distance the signal could travel while still remaining in-phase. The NRC invented a new technique that eliminated the need to directly compare the signals in real-time. Their technique used 2 inch Quadruplex videotape to record the signals along with a clock signal from an atomic clock. The clock signal allowed the two signals to be later compared with the same accuracy that had formerly required direct realtime connections. NRC funded the installation of identical instruments at the ARO and a smaller telescope at DRAO. Combining the signals would simulated a single 3,074 km diameter radio telescope.

Having learned that the Americans were also attempting a similar VLBI experiment, they tried to be the first to successfully use the technique. Their target for the experiment was quasar3C 273. Recordings were made into the early morning of April 17, 1967. DRAOs tapes and atomic clock were shipped to the ARO for comparison, and after a month of trying to get the data to "line up", on May 21 they succeeded. After a few more days they had made the first highly accurate measurement of the size of the quasar, showing it was less than 100 light years across, about 1/1000 the span of the Milky Way. Further experiments revealed the fact that 3C 273 had a distinct "jet".[5]

In 1968 the 150 ft (46 m) telescope was used in a geodesy experiment that measured the distance between the ARO and space-tracking telescopes in Prince Albert, Saskatchewan to 2143 km ± 20 m.[6] Other early experiments included a study of flare stars by Queen's University. It was also used by Alan Bridle and Paul Feldman in 1974 for the first SETI search to be carried out at the 1.35 cm wavelength, emitted by water molecules in space.[7]

The 46m Thoth telescope (left) and 11m telescope (right) viewed from the entrance road at the Algonquin Radio Observatory.

The original surface of the 150 ft (46 m) telescope consisted of a mix of aluminum mesh and plates. The mesh was almost transparent to wavelengths less than around a centimeter, and the plated area was not smooth enough to focus shorter wavelengths either. As attention in radio telescopy turned to shorter wavelengths, representing higher energy events, the ARO became less useful. After planning to resurface it so that it could operate at wavelengths as small as 3 mm, the NRC decided to close the ARO in 1987 and purchase a 25% share in the new James Clerk Maxwell Telescope, which would include a radio telescope that could operate at 0.3 to 2 mm.[5]

In 1988 the NRC invited the operators of the Hay River Radio Observatory in the Northwest Territories, the Interstellar Electromagnetics Institute (IEI), to relocate their SETI efforts to ARO. Due to budget cuts the NRC had been unable to use the ARO for research for some time, and were looking for low-cost projects that might be able to make use of the equipment. IEI jumped at the chance, and operated a SETI effort known as Project TARGET on the 18 m UofT telescope until 1991, when continuing budget cuts forced the NRC to cease operation of the site.

The continuing solar measurements, now used worldwide to predict communications problems due to sunspot activity, were turned over to DRAO. At first the DRAO instrument was made "prime", and then once operation was demonstrated, the original Ottawa instrument was moved to join it as a hot backup.[5]

The University of Toronto also operated their own 18 m telescope at the ARO for some time, after having moved it from the David Dunlap Observatory which proved to be too close to the growing Toronto area. The smaller University of Toronto antenna and the 32-dish solar observatory were both donated to project TARGET, and have since been relocated to a new site near Shelburne, Ontario.

The main ARO telescope was later operated by Natural Resources Canada and the Space Geodynamics Laboratory, CRESTech, who used the telescope in VLBI projects to measure the movements of continental plates in geodetic surveys.[8] They have made several upgrades to the main 150 ft (46 m) telescope after taking over operations, allowing it to track at higher speeds necessary to track satellites.[6]

The telescope was used in ongoing VLBI experiments carried out by a worldwide consortium supported by the HALCA satellite, producing a 30,000 km-baseline telescope. The system is driven by the S2 software developed at York University.

The observatory is also equipped with a hydrogen maser that maintains time standard stability to one part in 1015 in order to facilitate data correlation. The facility provides educational field schools for students from junior high to postdoctoral training programs including York University's space engineering field school. By appointment ARO is open to visitors.

The telescope is operated in a global network with other large radio telescopes around the world in order to create an interferometric array. By careful correlation of this data researchers hope to create a telescope aperture with a resolving power equivalent to the diameter of the Earth.[10] The observatory hosts the Long Wavelength Laboratory of the University of Toronto, Dunlap Institute for Astronomy & Astrophysics[11] and the Communications and Operations section of York University's Space Engineering Laboratory.[12]